Conventionally, there has been widely known an internal combustion engine equipped with an intake-valve control apparatus capable of controlling open and close timings (and the maximum lift amount) of the intake valves (see, for example, Japanese Patent Application Laid-Open (kokai) No. H6-317116). As is known, when the open and close timings of the intake valves are advanced, a period during which both the intake valves and the exhaust valves are maintained in an opened state (hereinafter referred to as an “overlap period”) is prolonged, whereby the efficiency of charging air into each combustion chamber can be increased by making use of the inertia of intake air when the engine is operated at high speed and/or under heavy load. When the efficiency of charging air into each combustion chamber increases, the cylinder pressure (combustion pressure) during the expansion stroke increases. Accordingly, when the engine is operated at high speed and/or under heavy load, as a result of advancing the open and close timings of the intake valves, the cylinder pressure during the expansion stroke becomes higher as compared with the case where the open and close timings of the intake valves are not advanced.
In the apparatus described in this document, a failure of the intake-valve control apparatus is detected on the basis of the above-described phenomenon. Specifically, when the engine is operated at high speed and/or under heavy load, there is calculated a difference between an average effective cylinder pressure during a period in which advancement of open and close timings of the intake valves is instructed and that during a period in which advancement of open and close timings of the intake valves is not instructed. When the difference is less than a predetermined judgment value, the open and close timings of the intake valves are determined not to have been changed as instructed, whereby a failure of the intake-valve control apparatus is detected.
However, as described above, only when the engine is operated at high speed and/or under heavy load, a clear difference is produced in cylinder pressure during the expansion stroke between the case where the open and close timings of the intake valves are advanced and the case where the open and close timings of the intake valves are not advanced. Accordingly, the apparatus described in the above-mentioned document has a problem in that a failure of the intake-valve control apparatus cannot be detected correctly in operation states other than the state where the engine is operated at high speed and/or under heavy load.
Further, the cylinder pressure during the expansion stroke may change stemming from various factors other than the overlap period. Therefore, the apparatus described in the above-mentioned document may fail to detect a failure of the intake-valve control apparatus correctly even when the engine is operated at high speed and/or under heavy load. Therefore, there has been desire for an apparatus which can detect a failure of the intake-valve control apparatus more accurately.